Window cleaners are exempt from CIS, not needed, check with the tax man, I'm sure you will find it is not compulsory.
I've done loads of builders cleans in the past and I've never, ever needed it.
If the construction firm tell you it is needed they are wrong.
Nothing wrong with having it, a good way of saving money, used enough it gets you a nice rebate at the end of the year.
Window cleaning is not considered to be part of the construction process.
but if you do have one, and you are the one being paid, any that you then further subcontract the work out too will not need it.
Ian